Solid-body actuators of the piezoelectric or magnetostrictive type have the advantage that the electric energy applied thereto may be converted into mechanical movement (that is, a dimensional change of the actuator body) without friction, play or wear with a high degree of efficiency. Since the excursion amplitude of the actuator which is in the order of magnitude of up to 1500 .mu.m is too small for a number of applications, for higher setting paths such an excursion amplitude has to be converted into a motion of larger amplitude. Ratio transmission systems have been found advantageous with which an increase of the setting path may be obtained by means of an incompressible medium (liquid). To ensure a long service life, however, it is of importance to avoid leakages as described, for example, in an article by Jendritza and Schroder in "Konstruktion 46" (1994, pages 376-380).